1. Field of the Invention
The present invention relates to a method and an apparatus for generating a tera-Hertz (THz) wave.
2. Description of the Related Art
A region of a far-infrared radiation or a sub-millimeter wave is positioned at a light wave-radio wave interface and so its field has been left undeveloped both in technology and application in contrast to the light wave and the radio wave, which have been developed in their own fields. This field of far-infrared radiation or sub-millimeter wave, however, has been more and more important in effective utilization of a frequency band in wireless communications, accommodation of ultra-high communications, environmental measurement by use of imaging or tomography utilizing properties of an electromagnetic wave in such a frequency band, and application to biology and medicine. Hereinafter, a far-infrared radiation and a sub-millimeter wave is called a xe2x80x9cTHz-wavexe2x80x9d or xe2x80x9cTera-Hertz Wavexe2x80x9d.
A THz-wave is difficult both to generate and to detect and in fact has been generated conventionally by use of (A) free-electron laser, (B) backward oscillator (BWO), (C) p-Ge laser, etc.
A free-electron laser is, in principle, capable of generating any wavelength of THz-wave but requires a long electronic bunch for its oscillation near 1 THz to result in a large-sized device measuring 10 meters or so, thus having a problem that is not only expensive but also inconvenient in use.
A BWO is excellent in its spectrum purity and useful in a band of several hundred GHz but has a problem of a rapid decrease in its tunability at a frequency higher than 1 THz.
A p-Ge laser requires for its operation an extreme low temperature, which needs cooling by use of liquid helium.
Those prior art THz-wave generating means, therefore, can be used in a laboratory but are all large sized and expensive or often inconvenient in use, so that they have not been practical or simple enough to meet the needs in a variety of researches for application.
To solve those problems of the above-mentioned prior art THz-wave generating means, there is reported in the following references by the present inventor et al. such a room-temperature THz-wave generating means that has tunability in a band of 1-2 THz and that can operate in a small-sized laser device:
Reference 1: Japan Patent Publication Laid-Open No. 9-146131.
Reference 2: Unidirectional radiation of widely tunable THz-wave using a prism coupler under non-collinear phase matching condition, 1997 American Institute of Physic, Aug. 11, 1997.
Reference 3: Tunable Terahertz-Wave Generation by Parametric Oscillation and Its Application, The Review of Laser Engineering, July 1998.
Reference 4: The THz-wave Parametric Generation Characteristics of MgO:LiNbO3, The Transactions of the Institute of Electronics, Information and Communication Engineers, April 2000.
FIG. 1 is an illustration for showing a principle for generating the THz-wave. In the figure, a reference numeral 1 indicates nonlinear optical crystal (e.g., LiNbO3), a reference numeral 2 indicates a pump wave (e.g., YAG laser), a reference numeral 3 indicates an idler wave, and a reference numeral 4 indicates a THz-wave.
When a pump wave 2 is infected in a constant direction into a nonlinear optical crystal 1 having Raman activity and far-infrared activity, the induced Raman effect (or parametric interaction) causes an idler wave 3 and a THz-wave 4 to be generated through an elementary excitation wave (polariton) of the material. In this case, the energy conservation law given by Equation 1 and the momentum conservation law (phase matching condition) given by Equation 2 are established among the pump wave 2 (xcfx89p), the THz-wave 4 (xcfx89T), and the idler wave 3 (xcfx89i). Note here that Equation 2 represents a vector relationship and the non-collinear phase matching condition can be expressed as given at the upper right in FIG. 1.
xcfx89p=xcfx89T+xcfx89ixe2x80x83xe2x80x83(1) 
xe2x80x83xcexap=xcexaT+xcexaixe2x80x83xe2x80x83(2)
Thus generated idler wave 3 and THz-wave 4 have a spatial spread and their wavelengths change continuously according to their emergent angles. The generation of the broad idler wave and THz-wave in this single-path arrangement is called THz-wave parametric generation (TPG).
Note here that a basic optical parametric process is defined as annihilation of one pump photon and simultaneous generation of one idler photon and one signal photon. When an idler wave or a signal light resonate and if the intensity of a pump wave exceeds a constant threshold, parametric oscillation occurs. Moreover, annihilation of one pump photon and simultaneous generation of one idler photon and one polariton are combined to constitute induced Raman scattering, which is included in parametric interaction.
However, a THz-wave generated in a THz-wave generator with such a single-path arrangement as shown in FIG. 1 is very faint and has a problem that its major part is absorbed in a nonlinear optical crystal when it goes through it by several hundred micrometers.
FIG. 2 is a configuration diagram of a THz-wave generator which solves this problem. As shown in it, by arranging a resonator for the idler wave 3 in a specific direction (angle of xcex8), it is possible to enhance the intensity of the idler wave 3. In this case, the resonator is comprised of mirrors M1 and M2 on which high-reflection coating is applied and is set on a rotary stage 5, thus enabling adjusting the resonator angle finely. Moreover, each of these two mirrors M1 and M2 is high-reflection coated only half of it in surface area so that a pump wave 2 may pass through the remaining surface area. In FIG. 2, a reference numeral 6 indicates a prism coupler for taking a THz-wave 4 out.
In the THz-wave generator shown in FIG. 2, when an incident angle xcex8 of the pump wave upon the crystal is changed over a certain range (e.g., 1-2xc2x0), an angle between the pump wave and the idler wave in the crystal is changed, thus changing also an angle between the THz-wave and the idler wave. This change in the phase matching condition provides the THz-wave with continuous tunability in a range of, for example, about 140-310 xcexcm.
As described above, a THz-wave generated in a THz-wave generator having such a single-path arrangement as shown in FIG. 1 is very faint and, in fact, its intensity was only 2 pJ/pulse even when a strong Nd:YAG laser with an excitation intensity of, for example, 45 mJ/pulse was used as the pump wave.
Furthermore, the THz-wave generator having such a resonator as shown in FIG. 2 can indeed enhance the intensity of a generated THz-wave a few times as high as that by the single-path arrangement but with a wider oscillation spectrum (e.g., about 15 GHz), thus finding less practicability in a variety of measurement applications.
Furthermore, this apparatus has a complicated mechanism for driving the rotary stage 5 for rotating the resonator and also a complicated process of adjusting the resonator itself.
The present invention has been devised to solve those problems. That is, it is an object of the present invention to provide a method and apparatus which can greatly increase the power of a THz-wave generated by parametric generation under a non-collinear phase matching condition in a nonlinear optical crystal and which can narrow a spectrum width of the THz-wave.
It is another object of the present invention to provide a method and apparatus that can provide a variable wavelength of a generated THz-wave and also which can hold its generation direction at mostly constant.
The inventors of the present invention confirmed first in the world such a phenomenon that in parametric generation under a non-collinear phase matching condition in a nonlinear optical crystal, by performing excitation by use of a laser beam of a single frequency and injection-seeding by use of a single-frequency laser beam on a stokes wave (idler wave), a spectrum width of a generated THz-wave can be narrowed down to almost the sum of spectrum widths of the pump wave and the injection-seeded wave, and the power of the THz-wave can be increased greatly. The present invention is dependent on such a novel finding.
That is, the present invention provides a THz-wave generating method by injecting a pump wave (2) into a nonlinear optical crystal (1) capable of parametric generation to thereby generate an idler wave (3) and a THz-wave (4) in such a direction as to satisfy the non-collinear phase matching condition, in which a single-frequency first laser beam (7) is used as the pump wave and another single-frequency second laser beam (8) is injection-seeded in a direction in which the idler wave is generated.
Furthermore, the present invention provides a THz-wave generator comprising a nonlinear optical crystal (1) capable of parametric generation, a first laser device (12) for injecting a single-frequency first laser beam (7) as the pump wave (2) into this nonlinear optical crystal, and a second laser device (14) for injection-seeding another single-frequency second laser beam (8) in a direction in which an idler wave is generated by the pump wave.
According to the method and apparatus of the present invention, another single-frequency second laser beam (8) is injection-seeded using the second laser device (14) in a direction in which the idler wave (3) is generated, thus enabling increasing the intensity of the idler wave in a nonlinear optical crystal than by a method of generating an idler wave only by the parametric interaction. It is experimentally confirmed that this method and apparatus can enhance the optical intensity of the idler wave (3) in this direction and also can greatly enhance the intensity of the generated THz-wave.
Furthermore, it is experimentally confirmed that the idler wave (3) enhanced in intensity by the second laser beam (8) has strong directivity and also both the first laser beam (7) and the second laser beam (8) are of a single frequency, thus enabling not only improving the directivity in the generation direction of the generated THz-wave (4) but also narrowing its spectrum width greatly.
According to a preferred embodiment of the present invention, the second laser device (14) is of a tunable type, by which the wavelength of the second laser beam (8) used in injection-seeding can be changed, thus changing the wavelength of the THz-wave.
According to this method and apparatus, it is also possible to change the wavelength of the THz-wave without rotating the provided rotary stage 5 according to the prior art. Therefore, not only the mechanism can be simplified but also a measuring system employing a THz-wave can be adjusted more easily.
Furthermore, preferably, the apparatus is provided with at least one prism coupler (16) arranged on the nonlinear optical crystal, which prism coupler is made of such a material that has a small variation of refractive index in a THz-wave band to thereby reduce a change in the radiation angle which accompanies a change in the wavelength of the THz-wave.
It was confirmed by analysis that silicon (Si), if employed as the material of the prism coupler, would bring about little change in the radiation angle of a generated THz-wave even when the wavelength of the THz-wave is changed over a range of 150-300 xcexcm. Therefore, it is possible to fix a direction in which the THz-wave is generated, and to further simplify the measuring system employing a THz-wave.
According to a second embodiment of the present invention, the nonlinear optical crystal (1) has, in configuration, reflection face (1a, 1b) at least partially on its both surfaces, by which faces (1a, 1b) are reflected the first laser beam (7) and the second laser beam (8) to thereby pass through in the nonlinear optical crystal a plurality of times, so that as the first laser beam (7) and the second laser beam (8) are reflected at one end face or both end faces of the nonlinear optical crystal to thereby pass through in the crystal, the power of the THz-wave is enhanced.
Therefore, by this method and apparatus, the first laser beam (7) and the second laser beam (8) can pass through in the same nonlinear optical crystal (1) a plurality of times, to thus intensify the THz-wave generated.
A third embodiment of the present invention comprises the resonator (18) comprised of one pair of mirrors (18a, 18b) which are high-reflection coated, for amplifying the idler wave (3).
According to this configuration, the mirrors (18a, 18b) of the resonator (18) can further intensify the idler wave, thus further increasing the intensity of the THz-wave generated.